Multicenter Trial of supplemental therapeutic oxygen for prethreshold retinopathy of prematurity. This is a randomized blinded multicenter study utilizing 24 centers. 880 infants who develop Prethreshold ROP will receive continuous pulse oximetry saturation monitoring and be randomly assigned to oxygen administration at one of two specified oxygen levels. "Conventional" with pulse oximetry of 89-94% saturation vs "Supplemental" with pulse oximetry of 96-99% saturation. Their ROP status will be measured and recorded prospectively, and the primary outcome variable will be the proportion of infants who progress to Threshold ROP in at least one eye by 3 months after their expected date of full term delivery. Secondary outcome measures include other ophthalmic and neonatal outcomes. The projected sample size is compatible with a 2-3 year enrollment.